It's really difficult at the moment - so few of the cheaper cages available and a lot less choice generally. My preference for a robo was a low barred cage with narrow 7mm bar spacing - the savic mickey 2XL but a) it's not available right now and b) it's expensive.
With a robo you do need something that is low - due to fall risk injury - they are so tiny. A barred cage also needs to have narrow enough bars or they can stuck. There are tank style cages available and if you're in the Uk the Duna Multy and maxi duna multy are good options.
The Duna Multy is available here
Cage Duna Multy | Welcome to Porton Garden Aquatic & Pets
If you're in the US, this is similar and a good option. It needs the hayrack taking out - that could be a hazard for a robo - the hayrack is meant for guinea pigs but the cage is far too small for a guinea pig. I'd take the big house out as well as it fills the cage up. Good size for a dwarf hamster though.
tamburino
OR you could make a bin cage. But tbh I think the Duna Multy is better value and a bin cage could cost as much to make.